There exist coordinate input apparatuses which are used to input coordinates to an input surface by using a pointer or finger to control a connected computer or write characters or graphics.
Conventionally, as coordinate input apparatuses of this type, touch panels of various methods have been proposed or become commercially available. These coordinate input apparatuses are widely used because a terminal such as a personal computer can easily be operated on the screen without using any special tool.
There are various coordinate input methods using, e.g., a resistive film or an ultrasonic wave. A method using light is also known, as described in U.S. Pat. No. 4,507,557. In this method, a retroreflecting sheet is arranged outside the coordinate input region. By illumination units which are arranged at the corners of the coordinate input region to illuminate an object with light and light-receiving units which receive light, the angles between the light-receiving units and a shielding object such as a finger that-shields light in the coordinate input region are calculated. On the basis of the detection result, the pointing position of the shielding object is determined.
Japanese Patent Laid-Open No. 2000-105671 or 2001-142642 discloses an apparatus in which a retroreflecting member is arranged near the coordinate input region, and the coordinates of a point where retroreflected light is shielded are detected.
In, e.g., the apparatus described in Japanese Patent Laid-Open No. 2000-105671, the peak of a light-shielded portion by a shielding object, which is received by the light-receiving unit, is detected by waveform processing such as differential. With this processing, the angle of the light-shielded portion with respect to the light-receiving unit is detected, and the coordinates of the shielding object are calculated from the detection result. In Japanese Patent Laid-Open No. 2001-142642, comparison with a specific level pattern is done to detect one end and the other end of a light-shielded part and the center of the coordinates is detected.
In U.S. Pat. No. 4,507,557, pixels of a RAM imager serving as a light-receiving unit are read out and compared by a comparator to detect a light-shielded portion. If a light-shielded portion having a predetermined width or more is present, the center (½ position) of the pixels at the two ends is detected. On the basis of the detection result, the coordinates of the shielding object are calculated.
Assume that a thing except a finger and, for example, a pointing rod or pen with a high surface reflectance is used as a pointer (shielding object). Alternatively, assume that the nail of the finger used as a pointer is coated with a high-reflectance material such as a manicure, or the input position of the finger is near the light-receiving unit. In such cases, irradiation light from the illumination unit may directly be reflected by the surface of the pointer, and the reflected light may be detected by the light-receiving unit. At this time, the reflected light from the pointer is also detected in addition to the reflected light from the retroreflecting member, which should originally be detected by the light-receiving unit. This may cause a detection error or degrade the coordinate calculation accuracy.